1. Field of the Invention
This invention relates to a driver circuit, specifically to a driver circuit to drive a light emitting device.
2. Description of the Related Art
A white light emitting diode (hereafter referred to as a white LED) has been used as back light for a liquid crystal display panel. The white LED has a VF (forward voltage) ranging from 3.2V to 3.8V. A voltage as high as VF is to be applied across an anode and a cathode of the white LED to drive the white LED to emit light. A power supply voltage ranging from 2.7V to 5.5V, for example, is required for the power supply voltage to the white LED driver circuit. Therefore, when a power supply voltage Vdd is too low, the power supply voltage has been boosted to 1.5 times of Vdd before being supplied to the white LED.
FIG. 6 shows such a driver circuit to drive the white LED. In the circuit shown in FIG. 6, an anode of the white LED 50 is provided with a boosted voltage of 1.5 Vdd from a 1.5 Vdd generation circuit 60, while a cathode of the white LED 50 is provided with a ground voltage Vss (0V) through a driver transistor 70. The 1.5 Vdd generation circuit 60 generates the voltage of 1.5 Vdd from the power supply voltage Vdd. The 1.5 Vdd generation circuit 60 is disclosed in Japanese Patent Publication No. 2001-231249.
However, since the 1.5 Vdd generation circuit 60 heavily uses P-channel type MOS transistors because of restrictions due to structuring of the circuit, there arises a problem that it requires a large patterning area for the driver circuit.